Volt-second synchronization for magnetic loads

ABSTRACT

A method and device for connecting a load to an AC power source is arranged to ensure that the volt-second ratings of magnetic devices in the load are not exceeded, in order to limit in-rush currents resulting from saturation of the magnetic devices. In the case where the load is being disconnected from a first AC source and connected to a second AC source, the volt-seconds of the load can be measured and/or calculated during disconnect, in order to delay connection of the load to the second AC source by an amount sufficient to prevent saturation of magnetic devices and thereby ensure volt-second synchronization of the sources.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a method of connecting a load to an AC powersource or sources, and in particular to a load connection method inwhich the load is connected to the AC power source based on the magneticsaturation characteristics, in volt-seconds, of the load, therebyminimizing the current in-rush caused by reduced impedance due tosaturation of magnetic constituents of the load during connection.

The invention also relates to a load disconnect/reconnect method inwhich the magnetic saturation characteristics of the load are measuredor determined during disconnection of the load from a first AC powersource, and used to minimize in-rush current during re-connection of theload to a second AC power source.

Finally, the invention relates to devices that implement theabove-mentioned volt-second based connection and disconnect/reconnectmethods.

2. Description of Related Art

The use of static or electromechanical devices to achieve phasesynchronization when disconnecting from and reconnecting to AC powersources has made a considerable contribution to the power quality andreliability in critical IT, MIS, and communications facilities. However,one problem that is not solved by current phase synchronization devicesis the inability of the distribution system to survive the high initialinflux of current drawn by down stream transformers and other magneticdevices when they saturate. This initial influx of current can tripupstream protective devices and/or initiate bypass in an upstream UPS.

The performance of transformers and other magnetic devices is defined bytheir B-H curve. The axes of the B-H curve are flux density (B) andmagnetic field intensity (H). The flux density is the integral of theapplied voltage and is therefore proportional to the volt-seconds of theapplied voltage. The magnetic field intensity is proportional to thecurrent.

The relationship between B and H is determined by the permeability ofthe magnetic device and this relationship is generally non-linear. Theslope of the B-H curve is inductance. At high levels of flux density(volt-seconds) the B-H curve flattens causing the slope of the B-H curveto approach zero. The knee of the curve is where the curve starts toflatten and the device core starts to saturate, i.e., the part at whichincreases in the input voltage do not increase the secondary voltageproportionally.

If the applied volt-seconds exceed the rated volt-seconds for a ½ cycleinterval, or if there is a volt-second off-set, the core saturates, thedevice impedance is reduced (core saturation), and large current flowsin the power system.

When a magnetic device is disconnected and connected between two powersources that are out of phase, the applied volt-seconds can be twice therated volt-seconds, causing a large influx of current. The currentin-rush can be up to 12 times the rated full load input current for thefirst half cycle. Only the source impedance and the magnetic devicewinding resistance and leakage impedance will limit the current, andtypically the upstream protective devices(s) will trip or open and causethe loss of the critical loads supported by the transformer.

SUMMARY OF THE INVENTION

It is accordingly an objective of the invention to provide a method ofconnecting a load to an AC power source that ensures that thevolt-second ratings of magnetic devices in the load are not exceeded,and that therefore limits in-rush currents resulting from saturation ofthe magnetic devices.

It is a second objective of the invention to provide a method ofconnecting a load to an AC power source that involves disconnecting theload from a first AC power source and delaying the reconnection of theload to a second AC power source so as to achieve phase synchronization,and yet that does not exceed the volt-second rating of magnetic devicesin the load.

It is a third objective of the invention to provide a method ofre-connecting a magnetic load to an AC source in which the currentinflux for the first half cycle can be reduced to 1.25 times rated fullload rather than the conventional current influx of up to 12 times therated full load input current.

It is a fourth objective of the invention to carry out phase andvolt-second synchronization between a disconnecting power source and aconnecting AC power source in a manner that is transparent to the load.

It is a fifth objective of the invention to provide a method ofre-connecting a magnetic load to an AC source following randomdisconnection of the magnetic device from the source without anyknowledge of the applied volt-seconds before disconnection, or a methodof initially connecting the magnetic load to the AC source, without highinflux of current.

It is a sixth objective of the invention to provide connect devices,transfer switches, and/or disconnect/re-connect devices that utilizesthe above-described method.

These objectives are accomplished, in the accordance with the principlesof a preferred embodiment of the invention, by a method of connecting anAC power source to a load in which the connection is accomplished overan interval that takes into account volt-second characteristics of aload. In the case of a load connect/disconnect device or transferswitch, the preferred embodiment carries out both phase synchronizationand volt-second synchronization of the source to the load, there-connection of the load to the AC power source being based on avolt-second determination made during disconnection of the load from afirst AC power source.

In order to ensure that the volt-seconds synchronization in thedisconnect-reconnect transition is carried out so as make the transitiontransparent to the load in static devices, the disconnect-reconnecttransition outage should be less than ½ cycle of the base frequency, andthe influx of current should not exceed 125% of rated current. To beload transparent in mechanical devices, the disconnect-reconnecttransition should not increase the normal transition time by more than ½cycle of the base frequency.

In accordance with the principles of an especially preferred embodimentof the invention, the volt-second synchronization takes no more thanthree ½ cycles of the reconnecting source, and is carried out accordingto the following method:

-   -   Assuming that the load is to be disconnected from source S1 and        is reconnected to source S2, first measure or calculate the        volt-seconds VSd from the disconnecting source last ½ cycle        cross-over to the point of load disconnection;    -   Based on the measured or calculated volt-seconds, calculate        delay intervals of up to three ½ cycles of the second source to        complete the synchronization of the second AC power source to        the first AC power source;    -   Gate semiconductor devices into conduction or close mechanical        contacts at the end of the calculated delay intervals following        disconnection from the first AC power source.

The delay intervals of the preferred method may be established bycalculating three delay times Td1, Td2, and Td3 based on the equalityVSd+VSc1+VSc2+VSc3=2*Aoc/Wc, as follows:

-   -   Td1 is the delay from the load disconnection point to        reconnection during the first ½ cycle of the connecting source        that occurs after disconnection. This time determines VSc1.    -   Td2 is the delay from the load disconnection to reconnection in        the second ½ cycle of the connecting source that occurs after        disconnection. This time determines VSc2.    -   Td3 is the delay from the first crossover of the third ½ cycle        to reconnection in the third ½ cycle of the connecting source        that occurs after disconnection. This time determines VSc3.    -   VSc1=volt-seconds of the first ½ cycle of the connecting source        after load reconnection to the end of the ½ cycle. VSc1 has a        positive sign if the voltage is positive.    -   VSc2=volt-seconds of the second ½ cycle of the connecting source        after load reconnection to the end of the ½ cycle. VSc2 has a        positive sign if the voltage is positive.    -   VSc3=volt-seconds of the third ½ cycle of the connecting source        after load reconnection to the end of the ½ cycle. VSc3 has a        positive sign if the voltage is positive.    -   Aoc=The peak value, in volts, of the sine wave form of the        connecting source.    -   Wc=omega=2*(PI)*Foc; Foc=connecting source frequency.    -   Aod=The peak value, in volts, of the sine wave form of the        disconnecting source.    -   Wd=omega=2*(PI)*Fod; Fod=disconnecting source frequency.

In the situation when a magnetic device is randomly disconnected from asource without any knowledge of the applied volt-seconds beforedisconnection, reconnection of the load to the source is preferablycarried out so that there is only 5% of the rated ½ cycle volt-secondsapplied for the first two ½ cycles. After the first two ½ cycles, 5%more volt-seconds is added for each subsequence two ½ cycles. After 20cycles the applied volt-seconds will be 100%. Since all magnetic devicehave at least a %5 over voltage rating, 5% added volt-seconds will notexceed the over voltage volt-seconds rating.

The invention may use any of the following categories of semiconductordevices:

-   -   Category 1 Load disconnect-reconnect devices using        semiconductors that can be gated into conduction with a pulse or        level applied to the semiconductor gate/base and that will        remain conducting until the pulses and/or level stops and an        external mechanism reduces the current flowing though the        semi-conductor to a specified small value, generally approaching        zero current. Time increments of ½ cycles are used in the text        to allow the use of Category 1 devices that may take part of the        next ½ cycle to disconnect after the applied voltage zero cross        over. Typically the devices take ⅙ of a cycle to disconnect        after the applied voltage zero cross over.    -   Category 2 Load disconnect-reconnect devices using        semiconductors that can be gated into conduction with a pulse        applied to the semi-conductor and that will remain conducting        until a second pulses stops conduction, with no external        mechanism required to reduces the current flowing though the        semiconductor.    -   Category 3 Load disconnect-reconnect devices using        semiconductors that can be gated into conduction with a voltage        level applied to the semi-conductor and that will remain        conducting until the voltage level is removed.

In addition to application to semiconductor devices, the method of theinvention may also be applied to mechanical disconnect re-connectsystems including electromechanical devices that use contacts todisconnect and reconnect the load, although it will be appreciated bythose skilled in the art that the algorithm defined in this inventiononly applies to electro-mechanical devices that can disconnect the loadand hold in a center state for a specified delay and then reconnect.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a single or multiple phase staticdisconnect-reconnect system that can utilize the volt-secondsynchronization method of the invention.

FIG. 2 is a block diagram of a single or multiple phase mechanicaldisconnect-reconnect system that can utilize the volt-secondsynchronization method of the invention.

FIG. 3 are waveform diagrams showing typical waveforms of a mechanicaldisconnect-reconnect device without volt-second synchronizaiton.

FIG. 4 are waveform diagrams showing waveforms of a mechanicaldisconnect-reconnect device with volt-second synchronization inaccordance with the principles of a preferred embodiment of theinvention.

FIG. 5 are waveform diagrams showing typical waveforms of a staticdisconnect-reconnect device with volt-second synchronization.

FIG. 6 are waveform diagrams showing waveforms of a staticdisconnect-reconnect device with volt-second synchronization inaccordance with the principles of a preferred embodiment of theinvention.

FIG. 7 is a flowchart showing the principal steps of a volt secondsynchronization method in accordance with the principles of theinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a static disconnect-reconnect system to which the method ofthe invention may be applied. Those skilled in the art will appreciatethat the system illustrated in FIG. 1 is exemplary in nature only, andthat the method of the invention may be applied to a variety of transferswitches and disconnect/reconnect devices.

The system illustrated in FIG. 1 respectively disconnects load 1 from ACpower source S1 and re-connects it to AC power source S2. It includes adigital controller 2, operator controls 3, switching circuitry 4controlled by signals received from the controller 2 and including oneor more pairs of semi-conductor devices connected in anti-parallel,switching circuitry 5 also consisting of one or more pairs ofsemi-conductor devices connected in anti-parallel and connected toreceive control signals from the controller 2. Current sensors 6 and 7are arranged to sample current output by the respective switchingcircuits 4 and 5, and to transmit the current sensing signals to thecontroller 2 via analog-to-digital converter 8. The voltage supplied tothe load is preferably detected by analog-to-digital converter 10 via afuse 11 and also supplied to the controller 2, which controls thesemi-conductor devices based on the detected current and voltagesamples, and the method described below.

The system illustrated in FIG. 2 is a mechanical disconnect-reconnectsystem that also can utilize the volt-second synchronizing method of theinvention, so long as the electromechanical devices 11 and 12 thatreplace semi-conductor devices 4,5 of FIG. 1 can disconnect the load andhold in a center state for a specified delay and then reconnect. Thecontroller 2, operator controls 3, current sensors 6,7,analog-to-digital converters 8,10, fuse 11 may be similar to thecorrespondingly numbered elements shown in FIG. 1, and the method ofcontrolling the controller 2 analogous to that used in connection withthe controller of FIG. 1, except as noted below.

FIG. 7 illustrates a method of volt-second synchronization according tothe preferred embodiment of the invention, in which the volt-secondsynchronization will take up to three ½ cycles of the reconnectingsource, as follows:

-   -   Upon disconnection of the load from source S1, and before        reconnection to source S2, the volt-seconds Vsd from the        disconnecting source last ½ cycle cross-over to the point of        load disconnection is measured or calculated (step 100). VSd has        a positive sign if the voltage is positive.    -   The delay times are then calculated (step 110). It may take        three ½ cycles of S2 to complete the synchronization of S2 to        S1. The total load outage time for semiconductor devices is ½        cycle or less.    -   Finally, the semiconductor devices are gated into conduction, or        mechanical contacts are closed, based on the calculated delay        times.

In a preferred embodiment of the invention, the delay times Td1, Td2,and Td3 are based on the equality VSd+VSc1+VSc2+VSc3=2*Aoc/Wc, where:

-   -   Td1 is the delay from the load disconnection point to the        reconnection in first ½ cycle of the connecting source that        occurs after disconnection, this time determines VSc1.    -   Td2 is the delay from the load disconnection to the reconnection        in second ½ cycle of the connecting source that occurs after        disconnection, this time determines VSc2.    -   Td3 is the delay from the first cross over of the third ½ cycle        to the reconnection in third ½ cycle of the connecting source        that occurs after disconnection, this time determines VSc3.    -   VSc1=volt-seconds of the first ½ cycle of the connecting source        after load reconnection to the end of the ½ cycle. VSc1 has a        positive sign if the voltage is positive and a negative sign if        the voltage is negative.    -   VSc2=volt-seconds of the second ½ cycle of the connecting source        after load reconnection to the end of the ½ cycle. VSc2 has a        positive sign if the voltage is positive.    -   VSc3=volt-seconds of the third ½ cycle of the connecting source        after load reconnection to the end of the ½ cycle. VSc3 has a        positive sign if the voltage is positive.    -   Aoc=The peak value, in volts, of the sine wave form of the        connecting source    -   Wc=omega=2*(PI)*Foc; Foc=connecting source frequency    -   Aod=The peak value, in volts, of the sine wave form of the        disconnecting source    -   Wd=omega=2*(PI)*Fod; Fod=disconnecting source frequency

In the above-described method, volt-second synchronization is based onthe summing of the positive and negative ½ cycles, with volt-secondsbeing synchronized during the total disconnect and reconnect transition.The lowest transition time is based on reconnection as quickly aspossible after disconnection.

Of course, the above method of determining the delay times may bemodified by using appropriate approximations, since the generalalgorithm for synchronization for producing low influx current canrequire a large amount of controller processing time. Approximationswhich do not require large amount of controller processing time aredescribed below in connection with various specific implementationexamples.

The three steps illustrated in FIG. 7 will now be described in moredetail:

1. Calculation or Measurement of Volt-Seconds During Disconnection (Step100)

The volt-seconds applied to the magnetic load during the half cyclebefore load disconnection may be measured or calculated by using currentsensors 6,7 to determine the direction of the current, voltage samplesobtained by A/D converters 8,10 to measure Aod and Wd and the varioustime periods, and the following calculation of the volt-seconds Vsd asfollows:

-   -   VSd=Aod/Wd*(−cos(Wd*Finish time)+cos(Wd*start time))    -   Aod=the peak value, in volts, of the sine wave form of the        disconnecting source    -   Wd=omega=2*(pi)*Fod; Fod=disconnecting source frequency.

In this implementation, the volt-seconds VSd of a partial ½ cycle of asine wave from initial zero cross-over to Tdis1 can be calculated asfollows:

VSd=Aod/Wd*(−cos(Wd*Tdis1)+cos(0))+VSde=Aod/Wd*(−cos(Wd*Tdis1)+1)+VSde  (Eq. 1)

-   -   Where:    -   Tdis1=is the time from the initial ½ cycle zero cross-over to        the load disconnection point. This time is data generated and        stored by the controller. VSde=the error due to measurement or        calculation which is somewhere constant and can be found by        controller learning algorithms. Look-up tables can be used if        VSde is not constant.

The S1 volt-seconds must be normalized to S2 volt-seconds with respectto differences in Aox and Wx amplitudes.   (Eq. 2)

VSdn=(Aoc/Aod)*(Fod/Foc)*VSd   (Eq. 3)

-   -   a. Aod=peak amplitude of the connecting source    -   b. Foc=Frequency of connecting source    -   c. VSdn=normalized VSd

Alternatively, the controller 2 can sample the ½ cycle voltage wave fromcross-over to time Tdis1 and then calculate the volt-seconds. Thesampled voltage is the instantaneous sum of the winding(s) voltage onthe respective phase, as follows (the number of samples in the ½ cyclesand the a/d converter will determine the accuracy):

VSd=volt-seconds=Tint* {ΣV ₁+(V ₂ +V ₃)/2+ . . . (V _(n−1) +V_(n))/2}+Vsde,   (Eq. 4)

-   -   Tint=sampling interval in seconds    -   V₁, V₂ . . . V_(n)=Sampled voltage amplitudes VSde=The error due        to measurement or calculation which is somewhere constant and        can be stored by controller learning algorithms. Look-up tables        can be used if VSde is not constant

The S1 volt-seconds must be normalized to S2 volt-seconds with respectto differences in Aox and Wx amplitudes.   (Eq. 5)

VSdn=(Aoc/Aod)*(Fod/Foc)*VSd   (Eq. 6)

-   -   d. Aod=peak amplitude of the connecting source    -   e. Foc=Frequency of connecting source    -   f. VSdn=normalized VSd

2. Calculation of Delay Times (Step 110)

The delay time calculations will be affected by the different types ofsystems to which the principles of the invention may be applied.Accordingly, the following description of the delay time calculationmethod includes several different cases:

-   CASE 1—General algorithms to reduce influx current for a single    phase of a multi-phase system or for a single phase system.    -   a. Connecting Source Leads the Disconnecting Source,        Semiconductor Devices

In this case the time delays for the first three ½ cycles of the loadreconnection are is given by the following:

VSdn+VSc1−VSc2+VSc3=2*Aoc/Wc,   (Eq. 7)

where:

-   -   a. Wc=omega=2*(pi)*Foc; Foc=connecting source frequency;    -   b. VSdn and VSc1 and VSc3 have the same sign; VSc2 has the        opposite sign;    -   c. VSdn is measured or calculated as shown above;    -   d. VSc1 should be as large as possible, i.e., Td1=0, so that the        S2 semiconductors are gated on as quickly as possible after the        S1 semiconductors are gated off; and    -   e. For values of Tdis1>=1/(2*Fod)−Tps, Td1 is ignored and VSc1=0        [where Tps=(phase shift betw sources)/(2*PI( )*Fo)].    -    Since category 1 semiconductors stop conducting when the load        current decreases below holding current and due to a non-unity        power factor, the cross-over point of the voltage waveform and        the crossover point of the current waveforms do not coincide.        Tdis2 is the time at which the category 1 semiconductors stop        conducting the second ½ cycle after being gated “on” in the        first ½ cycle. Other categories of semiconductor categories can        be gated off at the cross-over point in the voltage wave form.

Td3=(1/Wc)*A Cos [(Wc/Aoc)*(VSc3)−1];   (Eq. 8)

-   -   to keep transition times short; Td3 should equal zero; thus        VSc3=2*Aoc/Wc.

VSc2=−[2*Aoc/Wc.]+{VSdn+VSc1+VSc3}  (Eq. 9)

VSc2=Aod/Wd*(−cos(Wc*Tdis2)+cos(Wc*Tx)+2); Tx=Tps+Tdis+Td2−0.5/Foc  (Eq. 10)

-   -   a. Cos(Tdis2)=1 for all categories of semiconductor except        category 1.

Tx=(1/W2)*A Cos [(W2/Ao2)*VSc2−2+cos(Wc*Tdis2)].   (Eq. 11)

Td2=Tx+0.5/Foc−Tps−Tdis.   (Eq. 12)

If VSdn+VSc1−VSc2+VSc3>2*Aoc/Wc,   (Eq. 13)

-   -   then Td3 cannot be equal to zero, for full conduction of the        third ½ cycle, and Td3 can be calculated using the methods        above.

When Tdis1>=[Tav/(4)]−[Tps/2] to Tdis1<=[1/(Fod*2)]−[Tps/2], the loadoutage time is<=1/[Tav/2], (8.33 Ms for 60 Hz) and equation 12 reducesto equation 14, in which Td1 can be ignored; there is no reconnect inthe first ½ cycle of connecting source, Td3 is ignored i.e.VSc3=2*Aoc/Woc; and full conduction occurs during all of the third ½cycle:

Td2=Tav−2*Tdis1−Tps, where:   (Eq. 14)

-   -   Td2=the time delay in seconds from load disconnect (Tdis1) time        to load reconnect time;    -    The load will be reconnected to the connecting source in the        second ½ cycle of the connecting source, and therefore    -   VSc1=0;    -   Foc=frequency of connecting source;    -   Tdis1=load disconnect time in seconds measured from cross-over        of the ½ cycle to actual load disconnect; and

Tav=[1/(2*Fod)+1/(2*Foc)].

When Tdis1<=[Tav/(4)]−[Tps/2] or Tdis1>=[1/(Fod*2)]−[Tps/2] and equation12 is used to determine Td2. Equation 14 can allow load outage times upto Tav seconds (16.66 Ms for 60 Hz) when the phase shifts betweensources is small. Outages longer than Tav/2 seconds generally cannot betolerated.

In order to decrease load outage time, equations 8 to 12 must be used.The controller can solve the equations directly or look-up tables can beused.

b. Connecting Source Lags the Disconnecting Source, SemiconductorDevices

In this case, Td1 is ignored, there is no reconnect in the first ½ cycleof connecting source, and:

Td2=Tps   (Eq. 15)

Td1 and Td3 are assigned values to ensure no conduction in first ½ cycleof S2 and full conduction of third ½ cycle of S2.

The application of equation 15 will result in transition times<=[T/2];(<=8.333 Ms for Fo=60 Hz). If shorter transistion times are desired,equation 7 can be used.

-   CASE 2—low values of phase shift between the disconnecting source    and the connecting source; high speed disconnect-reconnect time    i.e., semiconductor devices

a. If the connecting source lags the disconnecting source by 15 degreesor less, a 2-4 millisecond transition time between disconnection andreconnection to another source will typically reduce the influx currentof a standard magnetic load from X15 to an X1.25 influx of current afterreconnection, which is generally an acceptable current influx rating.

b. Similarly, if the connecting source leads the disconnecting source by8 degrees or less, a 2-4 millisecond transition time betweendisconnection and reconnection to another source will typically give aX1.25 influx of current after reconnection, which is generally anacceptable current influx rating

This case can not be used with electro-mechanical devices, due to thefast transition times required.

-   CASE 3—Electro-mechanical devices to disconnect and reconnect the    load to S1 and S2.

Electro-mechanical devices typically use contacts to disconnect andreconnect sources to a load. The device must be capable of disconnectingthe load, holding the load disconnected from S1 for a determined lengthtime (center-delay) and then reconnect the load to S2.

The volt-seconds applied to the magnetic load during the ½ cycle beforeload disconnection can be measured or calculated using equation 1 or 2above.

Since the time required to disconnect or reconnect the load in anelectro-mechanical device is many times the sub-cycle delay time requirefor volt-seconds synchronization, the controller must measure and storecertain device parameters so that disconnect and reconnect times can bepredicted.

The following parameters must be recorded and stored for each disconnectand reconnect period:

-   -   Applied connect and disconnect voltage    -   Temperature    -   Number of disconnects and reconnects    -   Power factor of the load    -    During factory testing the voltage and power factor can be        varied so the controller with have initial knowledge.

a. If the connecting source lags the disconnecting source forelectromechanical devices meeting the above conditions:

Td2=N+Tps, where   (Eq. 16)

-   -   N=an integer>=the number of S2 cycles required to reconnect the        load. The controller predicts N based on measured parameters

b. If the connecting source leads the disconnecting source:

Td2=N+Tps+(1/fc)−(2*Tdis1),   (Eq. 17)

where

-   -   N=an integer>=the number of S2 cycles required to reconnect the        load. The controller predicts N based on measured parameters.

3. Re-Synchronization (Step 120)

When the volt-seconds before disconnect can be measured or calculated,re-synchronization simply involves connecting the second power sourceafter the appropriate delay time, as indicated above. However, when amagnetic device is randomly disconnected from a source without anyknowledge of the applied volt-seconds before disconnection, minimizationof the in-rush current is preferably accomplished by reconnection of theload to the source so that there is only 5% of the rated ½ cyclevolt-seconds applied for the first two ½ cycles. After the first two ½cycles, 5% more volt-seconds is added for each subsequence two ½ cycles.After 20 cycles (40½ cycles) the applied volt-seconds will be 100%.Since all magnetic device have at least a %5 over voltage rating, 5%added volt-seconds with exceed the over voltage volt-seconds rating.This procedure again uses the following parameters:

-   -   VSrs=Aoc/Wc*(−cos (Finish time)+cos (start time));        VSrs=volt-seconds of an ½ cycle sine wave    -   Aoc=the peak value, in volts, of the sine wave form of the        connecting source    -   Wc=omega=2*(pi)*Fc; Fc=connecting source frequency        The finish time is 1/(2*Fc); the end of the ½ cycle.        Furthermore:

VSrs=Aoc/Wc*(1+cos(Wc*Td));   (Eq. 18)

-   -   Td=delay period from the start of the ½ cycle

VSrs should be N*5%*2*Aoc/Wc;   (Eq. 19)

-   -   N ranges from 1 to 20 cycles

Td=1/Wc*A Cos [N*0.1−1]  (Eq. 20)

Td=1/Wc*A Cos [−0.9]=7.136 Ms for the first cycle   (Eq. 21)

Td=1/Wc*A Cos(1.8−1)=1.70 Ms for the eighteenth cycle   (Eq. 22)

Td=0 for the twentieth cycle   (Eq. 23)

To resynchronize, the controller would calculate or have a look-up tableto determine the delay for each cycle from the first full cycle afterreconnect is initiated until the load is full reconnected after thetwentieth cycle.

EXAMPLES

FIGS. 3 to 6 are waveforms generated during a disconnect/connect cyclefor various set-ups, as follows:

FIG. 3 shows typical waveforms of a mechanical disconnect-reconnectdevice without volt-seconds synchronization and with a 105 degree phaseshift between sources. The peak value of the current waveform beforedisconnection was 100 amperes, and the peak value at reconnection was1900 amperes. The top three waveforms in FIG. 3 (AN volts, BN volts, andCN volts) are the three phase primary voltages to a 225 KVA transformer.The bottom three waveforms (A Amps, B Amps, and C Amps) are the linecurrents to the transformer primary.

FIG. 4 shows typical waveforms of a mechanical disconnect-reconnectdevice with volt-seconds synchronization and with a 105 degree phaseshift between sources and a normal interval between disconnecting andreconnecting of 50 MS to 60 MS, depending on applied voltage timing. Theuse of the synchronization method of the invention reduced the influxcurrent to approximately zero, i.e., the transformer primary peakcurrent was the same before disconnection and after reconnection. Again,the top three waveforms in FIG. 4 (AN volts, BN volts, and CN volts) arethe three phase primary voltages to a 225 KVA transformer. The bottomthree waveforms (A Amps, B Amps, and C Amps) are the line currents tothe transformer primary.

FIG. 5 shows typical waveforms of a static disconnect-reconnect devicewith volt-seconds synchronization and with a 15 degree phase shiftbetween sources and a normal interval between disconnecting andreconnecting of 2 MS to 4 MS, while FIG. 6 shows typical waveforms of astatic disconnect-reconnect device with volt-seconds synchronization, a105 degree phase shift between sources, and a 3 MS to 7 MS delay. Again,the use of volt-second synchronization reduced the influx current toapproximately zero. The top three waveforms in FIGS. 5 and 6 (AN volts,BN volts, and CN volts) are the three phase primary voltages to a 225KVA transformer. The bottom three waveforms (A Amps, B Amps, and C Amps)are the line currents to the transformer primary.

Having thus described a preferred embodiment of the invention insufficient detail to enable those skilled in the art to make and use theinvention, it will nevertheless be appreciated that numerous variationsand modifications of the illustrated embodiment may be made withoutdeparting from the spirit of the invention, and it is intended that theinvention not be limited by the above description or accompanyingdrawings, but that it be defined solely in accordance with the appendedclaims.

1. A method of connecting a load to a source, comprising the steps of:determining delay intervals based on volt-second characteristics of aload; and connecting an AC source to the load following said delayintervals.
 2. The method of claim 1, wherein said load is disconnectedfrom disconnecting source S1 and connected to connecting source S2 aftersaid delay intervals, and wherein said volt-seconds are determined basedon current and voltage measurements during disconnection of source S1from said load. 3-27. (canceled)
 28. A device for connecting a load to asource, comprising: means for determining delay intervals based onvolt-second characteristics of a load; and means for connecting an ACsource to the load following said delay intervals.
 29. The method ofclaim 28, wherein said load is disconnected from disconnecting source S1and connected to connecting source S2 after said delay intervals, andwherein said volt-seconds are determined based on current and voltagemeasurements during disconnection of source S1 from said load.